1. Field of the Invention
The present invention relates to a laser apparatus in which laser light produced by a stable resonator is injected into the cavity of an unstable resonator, whereby the light output of the unstable resonator conforms to the spatial and temporal characteristics of the light from the stable resonator.
2. Description of the Prior Art
The fundamental components of a laser are a resonator, a gain medium, and an output extraction device. A laser resonator basically consists of two resonator reflectors between which light travels back and forth. The region between the two reflectors is termed the resonator cavity. A laser gain medium within the cavity amplifies the light as it repeatedly traverses the cavity.
A stable resonator is one in which the light beam maintains a limited diameter as it bounces back and forth between the two resonator reflectors. Most commonly, the output beam is extracted from the resonator cavity by making one of the resonator reflectors partially transmissive.
Stable resonator lasers are known for their ability to produce a light beam having high collimation and spatial coherence and having a smooth spatial intensity profile free of interference fringes, these characteristics being collectively known as good "beam quality". However, because of the limited diameter of the beam and the power density limitations of the laser gain medium, it is difficult to achieve high output power with a stable resonator laser.
An unstable resonator is one in which the diamter of the light beam increases progressively as it bounces back and forth between the two resonator reflectors. The portion of the beam whose diameter exceeds a certain value is generally extracted to form the output beam. Typical extraction techniques include deflecting the output beam away from the axis of the resonator cavity by an annular "scraper mirror" oriented at an angle to the axis, or else allowing the output beam to escape the cavity when its diameter exceeds that of one of the resonator reflectors.
Unstable resonator lasers generally are adapted to producing higher output power than stable resonator lasers, but generally cannot achieve the same degree of beam quality. Furthermore, tuning the wavelength of the laser beam is more difficult with an unstable resonator.
A known technique for operating an unstable resonator laser so as to achieve the desirable beam quality and spectral tunability of a stable resonator laser is "injection locking", which is the injection of the output beam from a stable resonator into the cavity of an unstable resonator in order to lock the unstable laser beam in conformity with the temporal and spatial characteristics of the stable laser beam.
One disadvantage of conventional systems for injection locking an unstable resonator laser is that they require two complete lasers and hence require two separate laser gain media, which significantly increases the cost of the system. Another disadvantage is that the two lasers must be excited with precise synchronization so that the stable resonator laser injects a light pulse into the cavity of the unstable resonator during the critical "build-up time" of the unstable resonator, that is, the time period after the initial excitation of the unstable resonator laser during which laser oscillations build up in the unstable resonator, before the oscillations become established at their natural, free-running (i.e., unlocked) value.
Conventional injection-locking systems typically admit the beam from the stable laser into the unstable laser cavity through a hole in the center of one of the two resonator mirrors of the unstable laser. A disadvantage of this system is that light diffraction at the perimeter of the hole degrades the spatial uniformity and collimation of the output beam by introducing interference fringes therein.
U.S. Pat. No. 3,622,907 to Tomlinson et al. discloses a related system in which a laser gain medium is enclosed by a pair of mirrors, one being concave and the other being flat near its center and convex elsewhere. The central region of the mirrors forms a stable laser oscillator, and the outer region forms a laser amplifier. A portion of the light beam generated in the central stable oscillator region is diffracted and thereby coupled into the surrounding amplifier region, from which the output beam is eventually extracted. A disadvantage of this system is that diffraction of the beam at the interface between the flat and convex regions of the second mirror introduces interference fringes which degrade the collimation and spatial uniformity of the output beam.